Vascular calcification is a hallmark of atherosclerosis, a major cause of mortality and morbidity in the United States. Vascular smooth muscle cells (VSMC) contribute significantly to the development of atherosclerosis and vascular calcification. Emerging evidence supports the concept that vascular calcification resembles the process of osteogenesis. Increased oxidative stress and production of reactive oxygen species (ROS) accelerate the progression of atherosclerosis and vascular calcification. However, the molecular mechanisms underlying oxidative stress-induced vascular calcification have not been fully elucidated. Hydrogen peroxides (H2O2), a key ROS produced by vascular cells, has emerged as an important mediator of intracellular signaling. We found that H2O2 induced VSMC calcification in culture, featuring increased expression of bone markers and downregulation of VSMC markers, concurrent with increased expression and enhanced activity of Runx2, and the key osteogenic transcription factor. Further, H2O2-induced VSMC calcification was inhibited in Runx2 deficient VSMC; while overexpression of Runx2 alone promoted VSMC calcification. Importantly, increased expression of Runx2 has been found in advanced atherosclerosis lesions, but not in normal vessels. Therefore, we conclude that Runx2 plays an important role in oxidative stress-induced vascular calcification. However, whether Runx2 expression contributes to onset or progression of atherosclerosis and vascular calcification is not known; and the mechanisms whereby Runx2 in regulating the process have not been defined. General Runx2 null mice and Runx2 C-terminal ablation mice are neonatal lethal due to defective bone formation, which precludes the characterization of the role of Runx2 in vascular calcification in vivo. In this proposal, we will generate a mouse model with SMC-specific ablation of Runx2 gene to determine the role of Runx2 in atherosclerosis and vascular calcification in vivo; and further elucidate Runx2-dependent molecular signals in regulating oxidative stress-induced vascular calcification. We hypothesize that oxidative stress induces Runx2 that is essential for vascular calcification. Two Specific Aims will be pursued to test our hypothesis: Aim 1: Characterize Runx2 Dependent Vascular Calcification In Vivo. SMC-specific Runx2 ablation mice with apolipoprotein E deficiency background will be generated and used to characterize the role of Runx2 in the progression of atherosclerosis and vascular calcification in vivo. Aim 2: Define Runx2 Dependent Signals in Oxidative Stress-induced VSMC Calcification. Runx2-dependent stages during oxidative stress-induced VSMC calcification, Runx2 functional domains responsible for VSMC calcification and gene regulation; and Runx2-regualted molecular signals in H2O2-induced vascular calcification will be characterized. Understanding the function of Runx2 in regulating oxidative stress-induced vascular calcification will provide important insights into the development of novel strategies and targets for successful therapeutic interventions for atherosclerosis and vascular calcification.